Talking is strongly discouraged, except of course when it comes to extra-ordinary circumstances like intense pain, etc. We're taught to develope our observational skills while the instructor is demonstrating technique, and that becomes difficult if we feel that we can just ask uke or someone else about the technique--it becomes a crutch.

I try not to speak on the mat except to acknowledge a senior student's comment, or to apologize if my throw sent somebody near someone else (how embarassing :/).